The present disclosure relates to a fixing device mounted in an image forming apparatus employing an electro-photographic method, such as a copier, a printer, a facsimile, and a multifunction peripheral (MFP) equipped with functions of these apparatuses, and in particular, the present disclosure relates to a fixing device capable of releasing nip pressure between a heating member and a pressure member, and an image forming apparatus including such a fixing device.
Conventionally, an image forming apparatus includes a fixing device for fixing a toner image transferred from an image carrier onto a recording material such as a sheet. Known examples of fixing methods employed in fixing devices include a roller fixing method using a heating roller and a pressure roller that rotate in contact with each other, and a belt fixing method using an endless fixing belt as a heating member. For example, in a fixing device employing the roller fixing method, a toner image carried on a sheet is fixed on the sheet by heating and pressing the toner image at a nip portion between a fixing roller and a pressure roller pressed against each other.
In such a fixing device, a sheet jam is sometimes caused where a sheet under conveyance is stuck in a nip portion between the heating roller and the pressure roller. Thus, it is necessary to provide a configuration for releasing nip pressure between a heating member and a pressure member to clear a sheet jam.
For example, there is known a fixing device that includes a pressure mechanism giving nip pressure to a nip portion between a pressure member and a heating member, and a pressure releasing mechanism varying the nip pressure given to the nip portion, the pressure releasing mechanism being an eccentric cam contacting a roller of a spring compression member included in the pressure mechanism, and having a variable distance from its rotation center to its outer peripheral surface.
There is also known a fixing device that includes pressed state switching means switching between a first state where a fixing roller (a heating member) and a pressure member are relatively pressed against each other and a second state where the pressed state is released, the pressed state switching means being an eccentric cam.